The term polyester refers generally to the group of synthetic resins that are polycondensation products of dicarboxylic acids with dihydroxy alcohols. The term unsaturated polyester resin, as used herein, designates a linear-type alkyd possessing carbon-to-carbon double bond unsaturation in the polymer chain. These unsaturated polyesters may be crosslinked and thus cured by reaction with monomers such as styrene or diallyl-phthalate, usually in the presence of a peroxide to form insoluble and infusible resins without the formation of a by-product during the curing reaction. Other types of polymer resins are also known which include carbon-to-carbon double bond unsaturation in the polymer chain, and which can also be crosslinked and cured, such as urethane acrylates, epoxy acrylates, and the like.
Because of their versatility and cost effectiveness, these resins possess broad commercial utility. Such utilities include, but are not limited to, low-pressure laminating; attractive and durable coatings for concrete, masonry, wood, plastic, wallboard, and metal; specialty resins targeted for synthetic marble, boat hulls, polymer concrete, mine-bolt resins, transfer molding, restorative dentistry, automotive body repair, and the like.
Tertiary aromatic amines are widely used as cure promoters or accelerators for unsaturated resins in the presence of peroxide initiators. Exemplary tertiary amines useful as cure promoters include, for example, N,N-dimethylaniline (DMA), N,N-diethylaniline (DEA), N-(2-hydroxyethyl)-N-methyl aniline, N-(2-hydroxyethyl)-N-ethyl aniline, N,N-bis-(2-hydroxyethyl)-m-toluidine, N-(2-hydroxyethyl)-N-[2-(2-hydroxyethoxy) ethylaniline, N,N-bis-(2-hydroxyethyl)-p-toluidine (HEPT), and N,N-dimethyl-p-toluidine (DMPT). However, cure rates for these types of compounds could be improved.
Further, as such uses demonstrate, polyester resins and other crosslinkable resins are often used in environments, i.e., outdoors, where temperatures cannot be easily controlled. Accordingly, it has become increasingly important to increase the cure rate of such polymer resins at low temperatures without detracting from the physical properties of the resins. The effectiveness of tertiary amine cure promoters such as those listed above, however, can fall off dramatically at low temperatures. Further, while the N,N-dimethyltoluidines are known as effective cure promoters at temperatures as low as 10.degree. C. to 15.degree. C., these compounds become much less effective at temperatures approaching or below 0.degree. C.
In addition, the lower dialkylanilines and toluidines can be toxic and can produce irritating and offensive odors; can be corrosive; and cannot sufficiently cure the polymer surface.